Abstract: The changing demographics of developed nations underscores the need for regenerative medicine approaches to combat the clinical and financial burden of degenerative diseases. The basic understanding of how tissues are normally maintained by their resident stem cells is, therefore, key for pursuing regenerative approaches. Though a great deal of knowledge has been gained through the use of traditional experimental approaches over the past two decades, limitations and drawbacks of these techniques have precluded us from gaining a complete understanding of stem cell function, particularly in the in vivo setting. The goal of my New Innovator proposal is to develop a novel experimental paradigm for the study of stem cell biology. In this model, individual stem cells in a population are uniquely and genetically tagged in situ and these unique genetic tags, or barcodes, can then be used to dynamically monitor individual stem cell activity, lifespan, and differentiation in highly complex populations over time. We believe that this model can give us an unprecedented highresolution picture of the inner workings of a complex and dynamic stem cell system, and allow us to answer long-standing biological questions. Our findings ultimately may uncover conserved mechanisms of stem cell maintenance that are perturbed in old age or other disease contexts. Though this proposal will address the biology of the normal and malignant hematopietic (bloodforming) stem cell compartments, the modular nature of our model makes easily adaptable to any tissue. Our model is also suitable, among other things, for the study of aging and immunological problems. Public Health Relevance: This proposal will allow us to understand how blood-forming stem cells function in an organism. The knowledge gained from these studies will allow us to understand how stem cell function is affected in conditions such as bone marrow failure syndromes, aging, and cancers of the blood.